1. Field of the Invention
This invention generally relates to a solder bumping process, and more particularly to a process for forming high bumps by using a thin photoresist layer.
2. Description of the Related Art
The solder bumping process is widely used in flip chip technology. This process comprises forming an under bump metallurgy (“UBM”) layer on the bonding pads of the wafer; forming solder bumps on the UBM layer; and connecting the wafer with the substrate via solder bumps. Because the coefficient of expansion of the chip is different from that of the substrate, solder bumps receive more shear stress when the gap between the chip and the substrate is small. Hence, this reduces solder bumps” mechanical reliability.
Therefore, higher solder bumps are needed to increase the distance between the substrate and the chip to reduce the shear stress.
FIGS. 1-2 show a prior art process of forming high bumps. Referring to FIG. 1, a passivation layer 104 is formed to cover the surface of the wafer 100 and to expose the bonding pad 102. A UBM layer 106 is formed to cover the passivation layer 104 and the boding pad 102. Then the patterned photoresist layers 108 and 110 are formed on the UBM layer 106. The solder material 112 is filled into the opening that is formed by the patterned photoresist layers 108 and 110. By increasing the thickness of the photoresist layers 108 and 110, the depth of the resulting opening becomes larger and more solder material 112 can be filled into the opening. The width of the opening of the photoresist layer 108 is around 100˜120 μm. The thickness of the photoresist layers 108 and 110 is around 100 μm.
Referring to FIG. 2, the photoresist layers 110 and 108, and a portion of the UBM layer 106 are removed. Then the solder bump 112a is formed after the reflow step.
In this prior art process, because the opening of each patterned photoresist layer is small, multiple layers of patterned photoresist layers are stacked to form deeper openings. Thus, more solder material can be filled into the opening. However, using the current photolithographic techniques to form a deep (100˜140 μm) and small opening (<100 μm) is very difficult, thus resulting in a lower reliability and yields.